Plastics are polymers. Polymers are chains of molecules. Each link of the chain is usually made of carbon, hydrogen, oxygen, and/or silicon. To make the chain, many links, are hooked, or polymerized, together with a chemical reaction requiring a heat source that is generated by burning of fossil fuels such as petroleum products, natural gas, etc.
To create polymers, petroleum and other petroleum products such as hydrocarbon based gases are heated under controlled conditions and broken down into smaller molecules called monomers. These monomers are the building blocks for polymers. Different combinations of monomers are generated and produce plastic resins with different characteristics, such as strength or molding capability. Plastics are typically divided in to two major categories: (1) thermosets; and (2) thermoplastics.
A “thermoset” is a polymer that solidifies or “sets” irreversibly when heated. Thermosets are useful for their durability and strength, and are therefore used primarily in automobiles and construction applications, adhesives, inks, and coatings.
A “thermoplastic” is a polymer in which the molecules are held together by weak bonds, creating plastics that soften when exposed to heat and return to original condition at room temperature. Thermoplastics can easily be shaped and molded into products such as milk jugs, floor coverings, credit cards, and carpet fibers.
Plastic resins are processed in several ways, including extrusion, injection molding, blow molding, and rotational molding. All of these processes involve using heat and/or pressure to form plastic resin into useful products, such as containers or plastic film.
Plastic polymers are made in combination with other elements such as chlorine, fluorine, silicon, nitrogen and oxygen contribute to the diversity of potential uses for plastics, but also complicates recycling efforts. For most applications, plastics do not mix well with other plastics.
In addition to the various elements mixed with hydrocarbons to produce different plastic polymers, various additives are introduced to enhance specific properties or merely to alter appearance such as coloring additives. For example, black plastic trays used in microwaves cannot be mixed with clear plastic water bottles for recycling even though they are made from the same type of plastic if the desired output is recycled plastics of the same type.
It has been estimated that plastics account for about up to 15% by weight and 25% by volume of municipal solid waste produced in the United States. Increasing amounts of scrap and waste plastics have created ever expanding disposal problems for both industry and society in general. The increased popularity of bottled water has led to a huge increase in the amount of plastic bottles appearing in the municipal solid waste stream. The amount of plastic bottles sent to landfills has increased so much that several cities on the west coast of the United States are considering bans on the sale of water in disposable plastic bottles.
Incineration, landfilling waste-to-energy and recycling are currently the main techniques used to dispose of plastics. However, there are many problems associated with disposing of plastics.
One problem is that it takes a large amount of energy to incinerate plastic and incineration process produces many products that are harmful to humans and the environment such as carbon monoxide, carbon dioxide, chlorine, and other hydrocarbons. These gases may also contribute to the global warming problem.
Another problem is placing plastics in landfills takes a large amount of energy and landfill space. It takes many gallons of gasoline to bury a ton of plastic with machinery such as bulldozers in a landfill. Landfill space is a scarce and becoming even more scarce due to environmental problems associated with storing municipal wastes.
Another problem is that waste-to-energy conversion using plastics is not very efficient. Typically the energy used to convert fossil fuels to plastic is lost when plastics are burned for energy since waste-to-energy combustion is a relatively inefficient means of energy recovery.
Plastic recycling is the process of recovering scrap or waste plastics and reprocessing the material into useful products. Plastics are recycled by grinding waster plastic, re-melting and re-processing it into recycled plastics.
To assist recycling of plastic items, the Plastic Bottle Institute of the Society of the Plastics Industry devised a scheme to mark plastic by plastic type. A recyclable plastic container using this scheme is marked with a triangle of three “chasing arrows”, which enclose a number giving the plastic type as a plastic resin identification code as is illustrated in Table 1.
TABLE 11. Polyethylene Terephthalate (PET or PETE) used for soft drink bottles, cooking oil bottles,peanut butter jars, etc.2. High Density Polyethylene (HDPE) used for detergent bottles, milk jugs, etc.3. Polyvinyl Chloride (PVC or V) used plastic pipes, outdoor furniture, shrink-wrap, water bottles,salad dressing and liquid detergent containers, etc.4. Low Density Polyethylene (LDPE) used for dry-cleaning bags, produce bags, trash can liners,food storage containers.5. Polypropylene (PP) used for bottle caps, drinking straws, etc.6. Polystyrene (PS) used for Styrofoam peanuts, cups, plastic tableware, meat trays, take-awayfood clamshell containers, etc.7. OTHER: Other - This plastic category, as its name of “other” implies, is any plastic other thanthe named those listed in 1-6 and used for certain kinds of food containers, Tupperware, andNalgene, etc. 
Recycling a ton of PETE plastic saves about as much energy as is stored in 197 gallons of gasoline. Recycling HDPE plastic saves slightly more, LDPE slightly less. The energy savings from recycling PET is about the same as the average for plastic.
However, there are also many problems associated with plastic recycling. Currently the main focus for recycling is grinding separated plastic types, re-melting and re-processing into other plastic materials. Such plastic materials, in general, are limited in use to low quality plastics such as decorative plastics or are used in small amounts as filler in other new non-recycled plastics.
There have been some attempts to solve some of the problems associated with recycling plastics. For example, U.S. Pat. No. 4,162,880, that issued to Cobbs et al. entitled “Plastic scrap recovery apparatus,” teaches “A scrap recovery system for recovering scrap material from plastic articles such as plastic bottles. The system comprises a hammer mill for breaking the articles into a heterogeneous mixture of chips, a combination separator and sorter for separating the plastic chips from foreign objects and sorting the plastic chips into batches of chips of discrete homogeneous plastic material, a novel melter for melting the batches of homogeneous chips, and a pelletizer for reforming the molten material into solid marketable pellets.
U.S. Pat. No. 4,882,073, that issued to Griffith, entitled “Method and system for recovery of plastics from a settling basin,” teaches A system for recovery of plastic material floating on the surface of water in a settling basin is disclosed. The system includes a transportable trailer having a hoist extendable from the trailer. Additionally, the trailer includes a floating boom structure extendable between the shoreline of the basin for dividing the basin into a first surface are a and a second surface area both containing floating plastic material. The trailer further includes a pump suspendable from the hoist for pumping the plastic material from the settling basin to a transportable container positioned on the shore of the settling basin. The pump includes an intake base that is positioned at a predetermined distance below the surface of the settling basin to aid in the operation of the system. The plastic recovery system of the present invention provides a method to quickly and efficiently recover plastic materials floating on the surface of the water while increasing the safety to the operator of the system during its operation.”
U.S. Pat. No. 5,022,985, that issued to Nugent entitled “Process for the separation and recovery of plastics,” teaches “Plastics are separated and recovered from mixtures containing plastics and other materials, by flotation in an aqueous dispersion, wherein the disperse phase comprises a substance such as for example calcium carbonate having an average mean particle size from about 1 micron to about 75 microns. The process is particularly useful for separating polyethylene and polyvinyl chloride from comminuted wire and cable scrap.”
U.S. Pat. No. 5,061,735, that issued to Zielinski entitled “Process for the separation of plastics,” teaches “Thermoplastic materials are separated and recovered, according to the present invention, utilizing a process wherein a mixture of the thermoplastic material to be recovered and one or more contaminants are simultaneously heated and agitated. The mixture is heated to the temperature at which the thermoplastic will adhere to itself, but at which the contaminant has not become tacky. Impacting thermoplastic particles agglomerate, while the contaminant particles do not adhere to other contaminant particles or to the thermoplastic particles. The resulting mixture is passed through a series of screens of increasing mesh size to separate the larger thermoplastic particles from the smaller contaminant particles.
U.S. Pat. No. 5,070,109, that issued to Ulick and Carner entitled “Recovery of hydrocarbon products from elastomers,” teaches “the method is disclosed for the recovery of hydrocarbon products from elastomeric products such as discarded vehicle tires and other rubber products. The elastomeric products are immersed in a liquid heat transfer medium and heated to a temperature in the range of from about 575 to about 600 degrees for a period of from about 0.5 to about 2.0 hours. The process produces a methane-containing gas product, a low boiling fuel oil fraction, a light fraction elastomeric hydrocarbon solid, a heavy fraction elastomeric hydrocarbon solid, and steel cord when steel belted radial tires are processed.”
U.S. Pat. No. 5,136,117, that issued to Paisley, et al. entitled “Monomeric recovery from polymeric materials,” teaches A method is described for the recovery of high yields of monomers from waste and scrape polymeric materials with minimal amounts of char and tar. The process involves pyrolysis in a circulating fluid bed (CFB). The polymer is heated to a temperature of about 650.degree.C. to about 1000.degree.C. at a rate of more than 500.degree.C./sec in less than two seconds. Heat is supplied to the CFB by a stream of hot sand heated in a separate combustor. The sand is also used as the circulating fluid bed material of the CFB. The process is essentially devoid of solid carbon char and non-monomeric liquid products.”
U.S. Published Patent Application No. 20060001187, published by Allen, et al. entitled “Multistep separation of plastics,” teaches “Multistep recycling processes for preparing recycled plastic materials. The processes feature a sequence of operations selected from the group consisting of preprocessing operations, size reduction operations, gravity concentration operations, color sorting, sorting by thickness, friction, or differential terminal velocity or drag in air, surface to mass control operations, separation processes enhanced by narrow surface to mass distributions, blending operations, and extrusion and compounding operations. Plastic-rich mixtures are subjected to the process, and one or more recycled plastic materials are collected as outputs of the sequence of processes.”
However, none of these solutions solve all of the problems associated with recycling plastics. It is desirable to have new methods for recycling plastics that can also recover the raw materials used to produce the plastics in the first place.